PROJECT ABSTRACT The major obstacle for HIV-1 eradication is the existence of a small but long-lived latent reservoir that is impervious to antiretroviral therapy. This reservoir persists largely through the natural ability of memory T cells to homeostatically proliferate in response to ?c-cytokines IL-7 and IL-15. In addition, HIV-1 also infects cells of the myeloid lineage, such as macrophages, a cell type that can penetrate protected tissues including the central nervous system, and precipitate a spectrum of neurological impairments that can persist in the setting of antiretroviral therapy. Recently, several studies have demonstrated that dasatinib, a tyrosine kinase inhibitor that is FDA-approved for the treatment of chronic myeloid leukemia, has three novel activities that can specifically impact HIV-1 infection and the persistence of latent reservoirs. The first novel activity of dasatinib is the ability to block homeostatic proliferation. Thus, in Aim 1 we test the hypothesis that dasatinib will block the homeostatic proliferation of latently infected T cells and thus prevent maintenance of the latent reservoir, resulting in a faster decay over time. We also propose to identify the tyrosine kinase(s) whose inhibition by dasatinib blocks homeostatic proliferation and hypothesize that such target(s) is different from the cognate targets of dasatinib (Src, c-Abl and c-Kit). A second activity of dasatinib is its ability to block infection by HIV-1 by an unusual mechanism that involves dephosphorylation and activation of SAMHD1, a potent viral restriction factor, both in T-cells and macrophages. The signaling pathway leading to dephosphorylation of SAMHD1 at threonine-592 downstream of dasatinib is unknown, but is clearly different from that downstream of IFN signalling. Because of the high potency of the inhibitory effect in both T-cells and myeloid cells, the dasatinib-inhibited pathway represents a compelling target for discovery (the focus of Aim 2) and development of novel therapeutics. A third activity of dasatinib with relevance to viral infection lies in its potent immunomodulatory activities. Dasatinib, extensively administered and studied to CML patients, induces an expansion of large granular lymphocytes (LGLs) with cytotoxic or NK phenotype, an increase in cytotoxic populations granzyme B+, as well as a potent immune response resembling natural responses against CMV. All these effects appear to contribute to the highly effective anti-leukemic effects of dasatinib. Thus, the third Aim of these studies will be to test the hypothesis that dasatininb will enhance innate cellular immune mechanisms against HIV-infected cells. In summary, we propose to investigate these novel and promising antiviral activities of dasatinib in the context of HIV-1 infection, with a focus on understanding the biological mechanisms and evaluating the potential for future clinical application. Dasatinib therefore represents a novel front in our fight to eradicate HIV-1.